1. Field of the Invention
The present invention relates to an audio apparatus that includes a switching amplifier and a switching power supply and controls the frequency of a clock signal that drives the switching power supply.
2. Description of the Related Art
Currently, an audio apparatus including a switching amplifier and a switching power supply is available. When combining a switching amplifier and a switching power supply, since clock signals that drive the two have different frequencies (also called carrier frequencies), interference (beat) occurs between the frequencies and falls down in an audible band, causing a big problem.
To suppress this, generation of clock signals of the switching amplifier and the switching power supply from the same source is considered. However, in the switching amplifier, in view of an influence on the audible band and an improvement in performance, the frequency of a clock signal is often set to a relatively high frequency (several hundred kHz or higher); on the other hand, in the switching power supply, in order to prevent an increase in switching loss, the frequency of a clock signal is set to a relatively low frequency. Thus, it is not realistic to set the same frequency for them.
To solve this problem, there is proposed a technique for suppressing occurrence of a beat by dividing the frequency of a clock signal that drives a switching amplifier at an arbitrary frequency division ratio and using a resultant frequency as the frequency of a clock signal that drives a switching power supply. With this technique, however, when power to an audio apparatus is turned on and in a state in which a power supply voltage from the switching power supply is not supplied to a clock generation circuit that generates a clock signal of the switching amplifier, a clock signal used to operate the switching amplifier cannot be generated. Thus, it is not also possible to generate a clock signal of the switching power supply by frequency-dividing a clock signal of the switching amplifier. Hence, there is a need to separately provide another power supply circuit (other than the switching power supply) for supplying, when power is turned on, a power supply voltage to a clock generation circuit for generating a clock signal of the switching amplifier, and cause the clock generation circuit to operate by a power supply voltage from another power supply circuit to generate a clock signal of the switching amplifier.